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Old 25th December 2008, 08:09 PM   #1
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Question oscilloscope turned guitar amp?

hey all,

i've recently come into posession of an old eico 460 tube 'scope, and i got the idea of reforming its components into a guitar amplifier. i figured i'd use a 6AU8 pentode/triode as the first preamp stage, into a quasi-Fender/Marshall/Vox tone stack, into a paralleled 12AU7 gain stage, and finally into a single-ended 6CB6 for a bit over 1W of output power. here are the drawings of what i've got so far:

power supply:
http://files.geekysuavo.org/golem/sec1-supply.png

thoughts:
1.) is the heater ground set up properly? i know that 100R resistors are used to create a pseudo-ground when no center-tap exists, but i'm pretty sure i made a mistake somewhere. would it be better to connect the center-tap between the 22k and 100k resistors and omit the 100R resistors altogether?

preamp section A:
http://files.geekysuavo.org/golem/sec2-preampA.png

thoughts:
1.) just keep in mind that since i'm using parts from the 'scope, most component values are ballparks of the perfect values.

tone stack:
http://files.geekysuavo.org/golem/sec3-tonestack.png

thoughts:
1.) since i'm using such large-value pots here, will the gain reduction be too large?
2.) the 2M/220k pot/resistor combination is in a law-fake setup for the gain dial.

preamp section B:
http://files.geekysuavo.org/golem/sec4-preampB.png

thoughts:
1.) this is the first time i've tried to figure out paralleling triodes, so there's probably something wrong here.

poweramp:
http://files.geekysuavo.org/golem/sec5-poweramp.png

thoughts:
1.) this is probably where i'll need the most changes...
2.) there must be a better way to create a lower anode voltage (127V) than a simple resistor divider. will the 4.7k resistor really dissipate 11.7W???

this will be a guitar amplifier, so i'm nowhere near the requirements of hi-fi, and since it's a first project, i really want to know if anything's going to just fry on me.

thanks!
~ brad.
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Old 26th December 2008, 01:43 AM   #2
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Brad-
I've built a bunch of amps, but not designed one from 'scratch', so can't comment on your schematics much.

If this is indeed your first tube project, my advice would be to use the power transformer and perhaps a few (very few) other parts from the scope and then copy a proven circuit.

However, it looks like you have put a fair bit of thought and work into your plan. It could be a lot of fun.

Re: heater supply- it looks like you have a center-tapped heater winding, so you don't need the 2 x 100R. If the voltage divider is to 'lift' the heater windings above ground, another way to do this is to grab the voltage from above the Rk (cathode resistor on the power tube)- lowers the parts count a bit.

BTW, another source of guitar output transformers is
Matt Sespaniak
Good guy to deal with.

Cheers
John
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Old 26th December 2008, 02:22 AM   #3
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Hello Brad,
I have had the same idea for about a year now, and let me tell you it's a wonder I haven't seen this on the net before. Maybe someone has succesfully done it though. I dont spend all my time on the net. Just 85% of it. although "succesfully" can be judged differently depending on what your goals are and who listens to it.

In any case, what you suggest, according to your schematics, seems quite doable and realistic. but I don'T like the diodes in the driver stage on the cathodes. it's just, well, you're mixing a fully tube design with solid-state, in the signal path! that's just... weird. I mean you want to take advantage of a vintage unit, a real tube scope, and I'm guessing, the original tubes, right? well then, do just that and don't modernize it.

I'm sorry. I don't want to give the impression I'm bashing your design here, I really like the fact you're "re-cycling" a tube scope into something equally as useful and cool! I'd just not put the diodes on the cathodes. But since they're really easy to bypass, then it wouldn't hurt to try. I'll take a guess and say that even though you are using them to bias the cathodes, they might add a bit of buzziness/harshness to your sound. why not just use a 5W resistor around 220ohms?
Also, why don't you just use the last stage of the vertical output to drive an output transformer? Thats' what I would do.

and yes, I have built a couple of tube amps, mainly for guitar. Although I wouldn't consider myself to knowlegeable about the theory of tubes. (I mean transconductance characteristics and charts and stuff like that). although I do apply the knowledge I find in BOOKS to my work. Books that were written back in the good old days!

cheers!

by the way, what guitar(s) do you play?
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Old 26th December 2008, 02:35 AM   #4
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ehh... this happens to me often. I forgot to say something (after I continued to read your post). OK concerning your power amplifier design and the question "is that resistor really going to dissipate 12W?", then Yes, best case! This is assuming you have no load connected to this voltage divider. Heck I would have probably made the same mistake also. Buthtink about it, your power amplifer needs current, and probably something like 25mA, too. so add that to the 50 or so milliamps already being drawn and you've got a good 75mA going through that resistor now! You no longer have 127volts either...

you either need a big Zener diode (like I said, solid-state and tubes, I find weird, although this would be in the power supply, I consider it not so bad, since the signal isn't really going through it). OR you need to just use a the 365 volts! you don't want to? you know what, I don't really like anything over 250V either... makes me nervous.

In any case, happy holidays, and happy designing!
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Old 26th December 2008, 04:54 AM   #5
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thanks for the help on the design you guys!

gain-wire, i play an old les paul, and don't do it any justice when i do. :P

your tip about the diodes is a good one. i guess i got lazy and just decided to use diodes, haha.

i guess i forgot that the HT will slump with the current being pulled from the tubes... i'll have to recalculate around that... but am i correct in thinking that the anode voltage of the 6CB6 will be about 2*127V=254V when using 127V HT? so wouldn't it just fry the tube if i used 365V instead of the 127V?

thanks again!
~ brad.
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Old 26th December 2008, 12:44 PM   #6
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A couple of suggestions Brad. Rewire the B+ standby switch out of the high voltage location it is in now and place it between the center tap of the power xfmer HV winding and chassis ground. This will add a safety factor.

The 6CB6 is not an audio output tube. If you are tyrying to re-use a 7 pin socket get a 6AQ5. That is good for much more power and will solve the B+ dropping resistor issue you have since it will run on the main B+ rail from your power supply. As a bonus the 6AQ5 has the same characteristics as the famous 6V6 loved for musical instrument amps for it's tone.

Neat idea. Are you going to put a 5 inch stage monitor speaker in the hole where the scope tube used to go? Parts Express has one for around $15 that ought to work well for guitar.
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Old 26th December 2008, 03:34 PM   #7
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thanks for the HT switching suggestion. the case wiring in the oscilloscope was a nightmare, and i wanted to start from scratch, so i built my own wooden chassis. that speaker idea would've been pretty sweet though!

as for the 6AQ5 suggestion: since the max Va(dc) of the tube is 250V, doesn't that still mean i need a quiescent anode voltage less than 250V? (hence some kind of divider circuit?) i don't need a high output power or anything, and i'd heard that the 6CB6 was a good general-purpose pentode which could be wired for low-power audio output, so i chose it... here are the other available tubes from the 'scope:

2x 6J6
1x 6AU8

thanks,
~ brad.
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Old 26th December 2008, 04:09 PM   #8
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hi Brad,
I think Bob has good suggestions here. I didn't even check the maxe plate voltage on the 6CB6, so I would have made you burn it (probably). Bob seems to know his stuff, I've scratched the surface of the theory. Heck I didn't even understand "quiescent anode voltage"!

will you re-use the knobs? what about the chassis and transformer, you're using the transformer, right?

cheers,

Owen
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Old 26th December 2008, 05:07 PM   #9
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wow... well i'd rather not melt my old resistors just yet, and i'd like to build this amp with as little non-scope components as possible, so how could i safely get ~125Vdc to the anode of the 6CB6?

an idea:

if i simply rectified the mains voltage (i know, it's dangerous and probably stupid) i would get 170Vdc (after rectification) quiescent anode voltage and 340V peak anode voltage. if i used a 16ohm speaker through the EDCOR OPT (8ohm/8kohm) i would have 16kohm load impedance, which would be high enough to keep the loadline below the 2W Pa(max).

would that work? :P

thanks for all the help,
~ brad.
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Old 26th December 2008, 05:30 PM   #10
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Gee, you are right that the plate voltage is still a bit low on the 6AQ5. I just pulled that out of my head from distant memory. If you aren't trying to re-use a 7 pin socket available in the original scope cabinet (a shame) your choices for an output tube are huge for a non-critical SE application like this. What do you have access to? For example. Almost any octal base TV horizontal output tube could serve well, in fact it doesn't have to be octal if you have a socket for it. Common octals were 6BQ5, 6DQ6, 6BG6. These usually have plate cap connections.

Most TV sweep tubes will need a resistive voltage divider to supply reduced Grid 2 (screen) voltage while the plate will handle your full B+ voltage available with ease.

Other tubes just off the top of my head would include 6L6, 6V6, 6W6, 6Y6, 6K6, 807, 2E26, 6146. 9 pin miniature include 6BQ5, 6CL6.

You could wire both internal triodes in parallel on any number of twin triodes. 6SN7GTB is common and would handle the job and the high voltage giving you a couple of watts.
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